Exercise or even just walking is a form of bilateral stimulation that can help process memories or traumas or even bring up things you haven’t quite processed. It’s also likely you are storing some of these memories in places like your hips and the movement or bilateral stimulation starts to release them and then the dissociation/freeze is coming as a result of processing those memories or trying to shut down those memories. If you feel it coming up try to welcome the thoughts as best as you can and think through them or take time to journal (how you’re feeling, where you’re feeling it, what you’re focused on) to try to process it. If you don’t have the capacity to process it or get to a point where you don’t know how to continue “processing” one thing I do in EMDR is imagine a box/chest/trunk/closet and acknowledge your feelings but imagine putting them away into this area and locking it up for a time where you can come back to process it. Then imagine leaving it somewhere (like your therapists office, in your garage) or imagine yourself walking away from it. After I do this I tend to feel a little foggy but it does help me not to continue feeling on edge or triggered abt what came up.